Forecasting Constraints on Non-Thermal Light Massive Relics from Future CMB Experiments (CMB-S4/Simons Observatory)

TL;DR

This paper uses Fisher forecasts to evaluate how future CMB experiments can constrain non-thermal light relic models, focusing on parameters like ff and Mff, and finds that certain parameters are tightly constrained while higher moments of the distribution are not.

Contribution

It provides the first detailed forecast of constraints on non-thermal light relics from upcoming CMB experiments, comparing different distribution models and analyzing parameter correlations.

Findings

ff is more tightly constrained for heavier, less abundant relics.

Uncertainties on Mff differ by a factor of rac{3}{2} between models.

CMB Stage IV data is insensitive to higher moments of the relic distribution.

Abstract

In this work we present Fisher forecasts on \textit{non-thermal LiMR} models for a CMB Stage IV-like experiment and the Simons Observatory -- particularly focusing on a model of inflaton/moduli decay giving rise to non-thermally distributed dark sector particles, and also comparing our results with those for sterile particles following the Dodelson-Widrow distribution. Two independent parameters, $\Delta N_\mathrm{eff}$ and $M_\mathrm{sp}^\mathrm{eff}$, influence linear cosmological observables. We find $\Delta N_\mathrm{eff}$ to be more tightly constrained (by a factor of $10$) for a less abundant, heavier LiMR which becomes fully non-relativistic around matter-radiation equality than a more abundant, lighter LiMR which becomes fully non-relativistic just after recombination. The uncertainties on $M_\mathrm{sp}^\mathrm{eff}$ differ by a factor of $\sim3$ between the two cases. Our analysis also reveals distinct parameter correlations: the phenomenological parameters $\{\Delta N_\mathrm{eff},M_\mathrm{sp}^\mathrm{eff}\}$ are found to be negatively correlated for the former case and positively correlated for the latter. We obtain similar projected uncertainties on the cosmological parameters (in either case) for both the inflaton/moduli decay and the Dodelson-Widrow models when the first two moments of the LiMR distribution function, related to the phenomenological parameters, are matched. Finally, by constructing a modified distribution that matches the first two moments of the Dodelson-Widrow but deviates maximally in the third moment, we demonstrate that CMB Stage IV data is not expected to be sensitive to higher moments of the distribution.